Drug Metabolism in Drug Design and Development Basic Concepts and Practice

Atmospheric gases (O 2 and N 2 ) and gas-phase solvent molecules initially react
with electrons from a corona discharge and form secondary reactant ions
(H 3 O+and (H 2 O)nH+) through ion/molecule reactions:

N 2 þe!N 2 þ:þ2e

N 2 þ:þ2N 2 !N 4 þ:þN 2

N 4 þ:þH 2 O!H 2 Oþ:þ2N 2

H 2 Oþ:þH 2 O!H 3 Oþþ:OH

H 3 OþþH 2 Oþ:!HþðH 2 OÞ 2

HþðH 2 OÞn 1 þH 2 Oþ:!HþðH 2 OÞn

These solvent ions then undergo ion/molecule reactions with the analyte (M),
giving rise to protonated molecular ions [M + H]+through a proton transfer
reaction (Horning et al., 1974).
It is interesting to note that ionization in APCI occurs mainly in the gas-
phase, while ionization in ESI occurs in solution. Therefore, different
compounds will show varying sensitivity and charge distributions between
the two techniques. Also, due to the large excess of gas-phase reagent ions
generated by the corona discharge and the lower degree of ion/molecule
reaction in the gas-phase, APCI is recognized as being less susceptible to ion
suppression effects and typically provides a wider dynamic detection range
than ESI (Dams et al., 2003; Hsieh et al., 2001). Higher flow rates are used with
APCI (1–2 mL/min) relative to conventional ESI (0.1–0.5 mL/min) in order to
generate large amounts of reagent ions and maximize the collision frequency
needed for ion/molecule charge transfer. Because of the need for thermal
desolvation, extremely polar and large molecules (MW>1000 Da) that may be
susceptible to thermal degradation, are not well suited for APCI. ESI is more
commonly used for the analysis of peptides, proteins, carbohydrates, and
oligonucleotides, while both ESI and APCI can be used for small molecule
pharmaceuticals (MW<600 Da) and drug metabolism studies (Brewer and
Henion, 1998).

11.2.2.3 Atmospheric Pressure Photoionization (APPI) APPI is a relatively
new API method developed by Bruins and coworkers in 2000 (Robb et al.,
2000). The APPI source is similar to the APCI source in that mobile phase is
vaporized using a heated nebulizer (350–500C) to generate a dense cloud of gas-
phase solvent and analyte molecules. The APPI source uses a krypton discharge
lamp (hv= 10 eV) to photoionize dopants added to the liquid effluent entering
the ionization source. Dopants (usually toluene or acetone) are routinely added
to increase ionization efficiency and to act as a charge carrier for ionizing trace
levels of analyte by charge transfer (Hanold et al., 2004).

LC/MS INSTRUMENTATION 323